In sins of a solar empire which character gives the best abilities

Your question seems to be very broad, are you talking battle-wise? Strategy-wise? Economic advantage? To answer you briefly I would just go on a genereal side of battle strategies and hope that this would answer of what you are looking for. Battle-wise: The faction of the Advent is I believe one of the best in the game especially in long term ones. Especially of the abilties on their Capital Ships such as the Vengeance and the Mothership which could take on readily a incredibly larger force. Since their mothership has the ability to propagate damage and the Vengeance battlecruiser has the return damage ability to instantly destroy ships that tend to focus fire on one particular ship. However, they are not that powerful early on in the game since most of their ships are very fragile and rely heavily on their shield upgrades. For fast games, I would pick the vasari, since their torpedo frigates are very deadly in picking out single targets able to bypass shields quite easily. Also if you would upgrade it into an area attack, this would be devastating since each shot fired by 50 or more so of these frigates could end a battle quite easily and their long range can destroy still approaching ships and those that are escaping. For the TEC, I'm not very fond of their strategies, and their only advantage is in sheer numbers and repair drones and their economic abilites. Other than that, they have a huge disadvantage over the other races. So I would suggest build quickly and destroy quickly since this is the only weakness of the other races. Allowing them to research their abilities could spell disaster for you.